Process and apparatus for the manufacture of carbon microballoons

ABSTRACT

This invention provides a stepped heating cycle for the pre-treatment of phenolic microballoons prior to carbonization thereof, wherein the heating cycle comprises the steps of sequentially: gradually elevating the temperature of the microballoons to a temperature in the range 100° C.–170° C.; holding the microballoons at the elevated temperature for 1–24 hours; and gradually cooling the microballoons. This invention also provides a heat-dissipation reactor ( 11, 21, 31 ) which comprises a walled reaction chamber having a bottom and no top, the reaction chamber being fitted with high thermal conductivity inserts. When used in accordance with this invention ( 61 ), the volume within the walls of the reaction chamber is charged with phenolic resin microballoons. In a preferred embodiment, the reaction chamber ( 11, 21 ) is subdivided into a plurality of subchambers by a vertical grid of aluminum plates ( 19, 29 ). In this embodiment, about half or more of the wall area of each subchamber comprises aluminum and a top edge ( 17 ) of the aluminum wall material communicates with atmosphere above the reaction chamber.

FIELD OF THE INVENTION

This invention relates to a pyrolytic process for the batch manufactureof hollow carbon microballoons. This invention also provides anapparatus that is especially advantageous for use in the process of theinvention.

BACKGROUND OF THE INVENTION

Carbon microballoons are employed as low density filler for thermalinsulating materials, for instance, polyimides.

In order to decompose a formed degradable material such as a phenolicresin microballoon into carbon and recover the carbon structure intact,the heating and cooling steps of the process must be controlled, in aninert atmosphere, as to temperature, temperature gradient, and time.

U.S. Pat. No. 4,229,425 describes a process in which a continuouslyheated oven is lined with a muffle comprising an elongated quartzceramic tube which is heated to raise its temperature to or above thecarbonizing temperature of preformed degradable microballoon material.The patent indicates that the degradable microballoon material may bepolymers of alkyd or phenol resins or polyurethanes.

The muffle tube glows and transfers heat by radiation to a graphite boatwhich contains the degradable microballoons to be pyrolyzed. The muffleis heated from the outside in an oxygen atmosphere, but the inside ofthe muffle is flushed with an inert gas such as nitrogen so that theboat containing the degradable microballoons experiences a continuouslychanging inert atmosphere. Once the muffle is heated, it is said to benecessary to keep it heated continuously, since it will crack if it isallowed to cool. Therefore the microballoons must be fed into the muffleand removed therefrom as quickly as possible so that the muffle is notseriously cooled by opening it at one end.

Dwell time in the muffle of the graphite boat containing themicroballoons is dictated by the necessity of heating the microballoonsslowly enough so that they do not simply burst as a result of risinginternal gas pressure. The walls of the microballoons are microporous,and gases can diffuse therethrough if the temperature of the batch israised at a slow enough rate. The boat not only limits the rate ofheating of the microballoons but also acts as a sacrificial materialwhich is attacked by oxidizing gases and moisture given off as themicroballoons are heated.

According to U.S. Pat. No. 4,229,425, the decomposition heating processtakes about four hours, after which time period one end of the muffle isopened and the boat is removed and immediately placed in a second,unheated chamber which is also provided with an inert atmosphere, sothat the microballoons and the boat are not oxidized before they cancool below the auto-ignition temperature of carbon microballoons in air.The second chamber also reduces the rate of heat loss through radiationso that the carbonized material cools at a rate which will preventcracking or deterioration thereof.

The carbon microballoons which result from decomposition are thenrecovered from the cooled boat, are screened in order to break up anyagglomeration of the particles, and are then immediately packaged inairtight containers to keep them out of contact with moisture in theatmosphere. These microballoons are filled with nitrogen at the timewhen they are removed from the cooling chamber as a result of theprocessing steps described.

U.S. Pat. No. 4,229,425 indicates that efforts to speed up the processby shortening the heating period of the microballoons has resulted in alower yield of intact carbon microballoons due to rupturing of thespherical form, cracking, oxidizing, etc. The patent claims a batchprocess for manufacturing carbon microballoons which includes the steps:

-   -   (a) heating a first chamber to a temperature which is above the        carbonizing temperature of the porous-walled heat carbonizable        microballoon precursor material and within the range        2000–3000° F. (1093–1649° C.) and continuously maintaining the        temperature at that level;    -   (b) enclosing a batch of microballoons within a graphite boat        which surrounds the microballoons except for small openings        sufficient to pass gases through the boat;    -   (c) containing and heating the boat and batch in the heated        first chamber for about 4 hours while flushing the chamber with        an inert gas and until the batch is carbonized to form        microballoons; and    -   (d) removing the heated batch from the first chamber and        immediately confining it in a second unheated chamber, and        flushing the second chamber with an inert gas until the batch        cools below its self-ignition temperature in air.

As compared to the process described in U.S. Pat. No. 4,229,425, thepresent process—which comprises an intermediate cure routine—providescarbon microballoons with less breakage and ultimately enables themanufacture of a better foam product.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings that accompany this application are presently solely forthe purpose of illustrating certain aspects of the present invention.They are not to scale and are not intended to limit the invention in anyway.

FIG. 1 is a cut away perspective sketch of an embodiment of theheat-dissipation reactor of the invention.

FIG. 2 is a top plan view of the heat-dissipation reactor embodiment ofthis invention shown in FIG. 1.

FIG. 3 is a top plan view of an alternate configuration for aheat-dissipation reactor in accordance with this invention.

FIGS. 4A and 4B are perspective views of a heat-dissipation reactorcharged with phenolic resin microballoons in accordance with thisinvention.

FIGS. 5A and 5B are perspective sketches of grid elements that may beused together to manufacture a heat-dissipation reactor in accordancewith this invention.

FIG. 6 is a cut away perspective sketch of an embodiment of theheat-dissipation reactor of the invention charged with phenolic resinmicroballoons.

SUMMARY OF THE INVENTION

The present invention provides a method for manufacturing carbonmicroballoons that show significantly reduced cracking as compared tocarbon microballoons produced by methods that do not make use of thepre-treatment prior to carbonization described herein. Carbonmicroballoons produced in accordance with the present inventionconsequently also provide substantial improvement in compressivestrength when used as fillers in composites.

This invention provides a stepped heating cycle for the pre-treatment ofphenolic microballoons prior to carbonization thereof, wherein theheating cycle comprises the steps of sequentially: gradually elevatingthe temperature of the microballoons to a temperature in the range 100°C.–170° C.; holding the microballoons at the elevated temperature for1–24 hours; and gradually cooling the microballoons. This invention alsoprovides a heat-dissipation reactor which comprises a walled reactionchamber having a bottom and no top, the reaction chamber being fittedwith high thermal conductivity inserts. When used in accordance withthis invention, the volume within the walls of the reaction chamber ischarged with phenolic resin microballoons.

A processing embodiment of the present invention is a stepped heatingcycle for the pre-treatment of phenolic microballoons prior tocarbonization thereof. This heating cycle comprises the steps ofsequentially: raising the temperature of the microballoons toapproximately 140° C. over a period of about 8 hours; holding themicroballoons at approximately 140° C. for about 10 hours; cooling themicroballoons over a period of about 5 hours to a temperature ofapproximately 40° C.; raising the temperature of the microballoons toapproximately 140° C., over a period of about 5 hours; holding themicroballoons at approximately 140° C. for about 10 hours; raising thetemperature of the microballoons to approximately 150° C. over a periodof about 1 hour; holding the microballoons at approximately 150° C. forabout 20 hours; and cooling the microballoons for about 5.5 hours toapproximately 40° C.

In accordance with another aspect of this invention, phenolic resinmicroballoons that have been pre-treated in this way are removed fromthe heat-dissipation reactor, placed into a graphite reactor, and heatedin an inert atmosphere to convert the phenolic microballoons into carbonmicroballoons. Thus, another embodiment of the present inventioncontemplates protecting microballoons from combustion by processing themin a heat-dissipation reactor made in accordance with this invention.

A heat-dissipation reactor of the present invention includes a walledreaction chamber having a bottom and no top. The reaction chamber issubdivided into a plurality of subchambers—for instance, by a verticalgrid of aluminum plates—so that about half/or more of the wall area ofeach subchamber is made of highly thermally conductive material.Generally, in use, the subchambers are charged almost fully withphenolic resin microballoons. However, the top edges of the highlythermally conductive wall materials extend above the charge ofmicroballoons and are in contact with the atmosphere above the reactionchamber.

While a grid of aluminum plates provides a convenient means to implementthe present invention, the reaction chamber may contain other sorts ofstructural elements composed of highly thermally conductive material todissipate heat. For instance, vertical tubing or pipes (i.e., hollowcylindrical configurations) made of aluminum or some otherheat-dissipating material may be used instead of a grid. In thisembodiment of the present invention, these structural elements arearranged so that no point of volume within the walls of said reactionchamber is further than about 5 inches from one of said structuralelements. As with the grid embodiments, the top edges of the highlythermally conductive structural element materials are generally allowedto communicate with atmosphere above the reaction chamber when thereaction chamber is charged with phenolic resin microballoons.

DETAILED DESCRIPTION OF THE INVENTION THE PROCESS

One aspect of the present invention provides a batch process formanufacturing carbon microballoons. The process of this inventionincludes the step of loading commercially available cured phenolic resinmicroballoons into a heat-dissipation reactor designed in accordancewith the present invention. Heat-dissipation reactors contemplated bythe present invention are described in detail hereinbelow. Theheat-dissipation reactor with its charge of phenolic microballoons isthen loaded into a furnace, where it is subjected to a stepped heatingcycle.

The stepped heating cycle that is used in the present inventioncomprises raising the temperature of the microballoons to approximately140° C. over a period of about 8 hours, holding at that temperature forabout 10 hours, cooling for about 5 hours to a temperature somewhatabove ambient temperature, e.g., about 40° C., again raising thetemperature of the microballoons to approximately 140° C., this timeover a period of about 5 hours, holding the temperature at approximately140° C. for about 10 hours, raising the temperature to approximately150° C. over a period of about 1 hour, holding at that temperature forabout 20 hours, and finally cooling for about 5.5 hours to close toambient temperature, e.g., about 40° C.

Once the phenolic microballoons have been pre-treated in this manner,they are ready to be removed from the heat-dissipation reactor, placedinto a graphite reactor, and heating in an inert atmosphere to convertthem into carbon microballoons. This aspect of the conversion ofphenolic microballoons may be carried out in a manner that is in generalknown to those skilled in the art. For instance, phenolic microballoonsthat have been pre-treated in accordance with this invention may becharged into a graphite reactor which is then placed into a furnacesupplied with an inert atmosphere, e.g., nitrogen gas.

Carbonization of the phenolic microballoons may be accomplished by astepped heating cycle, in which the microballoons are heated to 300° C.over a period of 5 hours, from 300 to 435° C. over a period of 6.75hours, from 435 to 650° C. over a period of 14.3 hours, from 650 to 710°C. over a period of 3 hours, and from 710 to 810° C. over a period of 1hour, at which point the temperature is maintained at 810° C. until thephenolic microballoons are completely converted into carbonmicroballoons.

The newly formed carbon microballoons may be heat-treated by heating thegraphite reactor containing them in the furnace supplied with an inertatmosphere to 1800° C. and maintaining them at that temperature for 4hours. The carbon microballoons may then be removed from the furnace,cooled to ambient temperature, and subsequently used as low densityfiller for thermal insulating materials, such as polyimides.

The Reactor

Another aspect of the present invention is a heat-dissipation reactor,which may be employed to carry out the process of this invention. Theheat-dissipation reactor of this invention is illustrated in FIGS. 1–3.The heat-dissipation reactor (11, 21, 31) comprises a walled reactionchamber (15, 25, 35) having a bottom and no top. This reaction chambercan be made from metals such as steel or from alloys. One material whichhas been found to be useful for making the bottom and sidewalls of thereaction chamber is inconel, an iron-nickel alloy. The reaction chambercontains structural elements (19, 29, 39) composed of highly thermallyconductive material.

Aluminum is the preferred highly thermally conductive material, but anymaterial which has sufficient thermal conductivity, to remove enough ofthe heat generated by exothermic processes occurring within reactants inthe reaction chamber so that they do not spontaneously oxidize, may beemployed. Copper, graphite, and carbon-carbon composites could also beused as the highly thermally conductive material, as could any solidphase material having a thermal conductivity higher than 10 watts/meterK. Aluminum, for instance, has a thermal conductivity of approximately220 watts/meter-K.

In accordance with this invention, these highly conductive structuralelements are arranged so that no point of volume (13, 23, 33) within thewalls of said reaction chamber is further than about 5 inches from oneof said structural elements (19, 29, 39). This will ensure that thephenolic microballoons that fill the volumes within the reaction chamberare not too far removed from the highly conductive structural elementsto dissipate heat to them. Also, in order to dissipate heat, the topedges (17) of the highly thermally conductive structural elementmaterials communicate with atmosphere above the reaction chamber. Someheat is also conducted by the highly conductive structural elements intothe walls and bottom of the reaction chamber.

Referring to FIGS. 1 and 2, a preferred heat-dissipation reactor (11,21) comprises a walled reaction chamber (15, 25). The walls of thereaction chamber may be of any convenient dimensions. They may, forexample, range from 0.1 to 0.25 inches in thickness, and be from 24 to40 inches in length, 15 to 25 inches in width, and 10 to 20 inches indepth. The reaction chamber is subdivided into a plurality ofsubchambers, for example by a vertical grid of aluminum plates. Wherethe walls of the reaction chamber are 0.140 inches thick and measure34.675 inches in length, 19.25 inches in width, and 13.25 inches indepth, the reaction chamber may be fitted with a grid composed, forexample, of two 34-inch long aluminum sheets crossed at right angles bythree 18.625-inch long aluminum sheets, the aluminum sheets being 0.625inches thick and 12 inches wide. This configuration provides reactionsubchambers that are approximately 8 inches long, 6 inches wide, and 12inches deep. In this embodiment, about half (that is, 2 out of 4) ormore (that is, 3 out of 4 or 4 out of 4) of the wall areas of eachsubchamber is formed of aluminum. This is because, for the outercorners, 2 out of 4 subchamber walls are highly conductive, for theremaining exterior subchambers, 3 out of 4 walls are highly conductive,and for the internal subchambers, 4 out of 4 walls are highlyconductive.

FIGS. 4A and 4B are perspective views of a heat-dissipation reactorcharged with phenolic resin microballoons in accordance with thisinvention. As can be seen in FIGS. 4A and 4B, the phenolic resinmicroballoons are more or less evenly distributed within the volumesdefined by the 15 subchambers into which the reactor volume is dividedby the grid, and the top edge of the grid of highly conductive materialis edxposed to the atmosphere above the reactor.

FIGS. 5A and 5B are perspective sketches of grid elements that may beused together in the manufacture of a heat-dissipation reactor inaccordance with this invention. In FIG. 5A, bottom element 51 compriseshighly conductive structural element 59. In FIG. 5B, top element 52comprises highly conductive structural element 58. To make aheat-dissipation reactor such as that depicted in FIG. 4, four topelements 52 are interlocked with two bottom elements 51 to form a grid,and the grid is located within a walled reaction chamber. In this way,top edges 57, indicated in FIG. 5B, will be exposed to the atmosphere atthe top of the heat-dissipation reactor.

FIG. 6 illustrates a charged heat dissipation reactor embodiment of thepresent invention. Charged reactor 61 is similar to that depicted inFIGS. 1 and 2. In the embodiment of this invention depicted in FIG. 6,the volumes within the walls of the reaction chamber (including, e.g.,points 13 and 23 in FIGS. 1 and 2, respectively) are charged withphenolic resin microballoons 66.

Comparative Example

Approximately 40 pounds of phenolic microbeads are loaded into areaction chamber having four walls and a bottom but no top. The reactionchamber is made of inconel alloy 0.140 inches in thickness and measures34.675 inches in length, 19.25 inches in width, and 13.25 inches indepth. The reaction chamber with its phenolic microbeads is placed intoa furnace, and the furnace is heated to 140° C. using a stepped cycle,as follows: the temperature of the furnace and its contents is raisedfrom 30 to 100° C. over a period of 4 hours, then is raised from 100 to140° C. over a period of 4 hours, then is held at 140° C. for 10 hours,and finally is cooled to 40° C. over a period of 5 hours. Upon openingthe furnace, it is observed that the phenolic microbeads in the reactionchamber have reached a temperature above 300° C. and ignited on contactwith air.

EXAMPLE OF THE INVENTION

Approximately 40 pounds of phenolic microbeads are loaded into areaction chamber having four walls and a bottom but no top. The reactionchamber is made of inconel alloy 0.140 inches in thickness and measures34.675 inches in length, 19.25 inches in width, and 13.25 inches indepth. The reaction chamber is fitted with a grid composed of aluminumsheets 0.625 inches in thickness and having a depth of 12 inches. Thegrid is made up of two 34-inch lengths of aluminum and three 18.625-inchlengths of aluminum, arranged to provide isolation volumes approximately8 inches long, 6 inches wide, and 12 inches deep. The charge of phenolicmicrobeads is distributed more or less evenly among the twelve isolationvolumes that are formed by the grid within the reaction chamber. Thegridded reaction chamber with its phenolic microbeads is placed into afurnace, and the furnace is heated to 140° C. using a stepped cycle, asfollows: the temperature of the furnace and its contents is raised from30 to 100° C. over a period of 4 hours, then is raised from 100 to 140°C. over a period of 4 hours, then is held at 140° C. for 10 hours, andfinally is cooled to 40° C. over a period of 5 hours. Upon opening thefurnace, it is observed that the phenolic microbeads in the reactionchamber did not combust and were ready to be carbonized.

1. A batch process for manufacturing carbon microballoons, comprisingthe steps of: providing cured phenolic resin microballoons; graduallyelevating the temperature of the microballoons to a temperature in therange 100° C.–170° C. in a heat-dissipation reactor; holding themicroballoons at the elevated temperature for 1–24 hours; graduallycooling the microballoons; and subsequently removing the phenolicmicroballoons from the heat-dissipation reactor, placing them into agraphite reactor, and heating them in an inert atmosphere to convert thephenolic microballoons into carbon microballoons.
 2. A stepped heatingcycle for the pre-treatment of phenolic microballoons prior tocarbonization thereof, wherein the heating cycle comprises the steps ofsequentially: raising the temperature of the microballoons toapproximately 140° C. over a period of about 8 hours; holding themicroballoons at approximately 140° C. for about 10 hours; cooling themicroballoons over a period of about 5 hours to a temperature ofapproximately 40° C.; raising the temperature of the microballoons toapproximately 140° C., over a period of about 5 hours; holding themicroballoons at approximately 140° C. for about 10 hours; raising thetemperature of the microballoons to approximately 150° C. over a periodof about 1 hour; holding the microballoons at approximately 150° C. forabout 20 hours; and cooling the microballoons for about 5.5 hours toapproximately 40° C.
 3. A batch process for manufacturing carbonmicroballoons, comprising the steps of: providing cured phenolic resinmicroballoons; subsequently loading said microballoons into aheat-dissipation reactor; subsequently placing the loaded reactor into afurnace; subsequently raising the temperature of the furnace and itscontents to 100° C. over a period of 4 hours; subsequently raising thetemperature of the furnace and its contents from 100 to 140° C. over aperiod of 4 hours; subsequently holding the temperature of the furnaceand its contents at 140° C. for 10 hours; subsequently cooling thetemperature of the furnace and its contents to 40° C. over a period of 5hours; subsequently raising the temperature of the furnace and itscontents to 100° C. over a period of 1 hour; subsequently raising thetemperature of the furnace and its contents from 100 to 140° C. over aperiod of 4 hours; subsequently holding the temperature of the furnaceand its contents at 140° C. for 10 hours; subsequently raising thetemperature of the furnace and its contents from 140° C. to 150° C. overa period of 1 hour; subsequently holding the temperature of the furnaceand its contents at 150° C. for 20 hours; subsequently cooling thetemperature of the furnace and its contents to 40° C. over a period of5.5 hours; and subsequently removing the phenolic microballoons from theheat-dissipation reactor, placing them into a graphite reactor, andheating them in an inert atmosphere to convert the phenolicmicroballoons into carbon microballoons.
 4. A stepped heating cycle forthe pre-treatment of phenolic microballoons prior to carbonizationthereof while protecting said phenolic microballoons from combustion,comprising the steps of sequentially: charging the phenolicmicroballoons into a heat-dissipation reactor which comprises a walledreaction chamber having a bottom and no top, said reaction chamber beingsubdivided by highly thermally conductive structural elements into aplurality of subchambers, wherein about half or more of the wall area ofeach subehamber comprises a highly thermally conductive material andwherein a top edge of said highly thermally conductive wall materialcommunicates with atmosphere above said reaction chamber; raising thetemperature of the microballoons to approximately 140° C. over a periodof about 8 hours; holding the microballoons at approximately 140° C. forabout 10 hours; cooling the microballoons over a period of about 5 hoursto a temperature of approximately 40° C., wherein said microballoons areprotected from combustion by said heat-dissipation reactor.
 5. A steppedheating cycle for the pre-treatment of phenolic microballoons prior tocarbonization thereof while protecting said phenolic microballoons fromcombustion, comprising the steps of sequentially: charging the phenolicmicroballoons into a heat-dissipation reactor which comprises a walledreaction chamber having a bottom and no top, said reaction chambercontaining structural elements composed of highly thermally conductivematerial, wherein said structural elements are arranged so that no pointof volume within the walls of said reaction chamber is further thanabout 5 inches from one of said structural elements and wherein topedges of said highly thermally conductive structural element materialscommunicate with atmosphere above said reaction chamber; raising thetemperature of the microballoons to approximately 140° C. over a periodof about 8 hours; holding the microballoons at approximately 140° C. forabout 10 hours; cooling the microballoons over a period of about 5 hoursto a temperature of approximately 40° C., wherein said microballoons areprotected from combustion by said heat-dissipation reactor.